What is the difference between kW and kWh for EV charging?

Kilowatts (kW) measure power: how quickly electricity is being delivered. Kilowatt-hours (kWh) measure energy: how much is delivered or stored. A constant 1 kW for an hour equals 1 kWh [S1]. Use power to assess charging speed and energy to calculate a session’s electricity cost. Your site needs both measures. Checked October 2026.

Last checked 16 October 2026

What is the difference between kW and kWh?

Your electric vehicle (EV) proposal needs two different units. A kilowatt (kW) measures power. A kilowatt-hour (kWh) measures energy. National Grid’s table of units defines a kilowatt-hour as the energy delivered by a constant kilowatt over an hour [S1].

Use kW when asking how quickly charging can happen. Use kWh when asking how much electricity a session uses. A battery capacity is an amount of energy, while a charger rating is a rate of delivery. Those values cannot be compared as though they describe the same thing [S1], [S3].

Think of filling a tank: the flow rate resembles power and the quantity collected resembles energy. The comparison helps with the units, although charging a vehicle is more complicated than filling a tank. The vehicle itself can limit the rate at which it accepts electricity [S3].

These unit distinctions follow the definitions in National Grid’s glossary and the electricity measurement explanation from the US Energy Information Administration. The physical units are the same in the UK [S1], [S2].

How do you calculate kWh from charger power?

Multiply average power by time in hours. If power varies, use the average over the period, rather than the rating printed on the charger. The principle is energy equals power multiplied by time [S2].

Here is an illustration, not a quote or an observed session. Assume constant delivery, compatible vehicles and no losses. All the inputs below are chosen for arithmetic, not offered as typical usage.

The third row is a useful check: half an hour is 0.5 hours, not 0.30 hours. Convert minutes to hours by dividing by 60. For a changing charging rate, use recorded session energy rather than multiplying the headline charger rating by the time the cable was connected.

You can reverse the calculation. Assume you want to add 28 kWh at an average 7 kW. The arithmetic is 28 ÷ 7 = 4 hours. The charging-time guide explains why a real booking needs more information than this ideal calculation [S3].

How do kWh turn into a charging bill?

Multiply the energy charged for by the price per kWh. Keep the energy purchase and the driver sale separate. The unit price applies to energy, not to the maximum kW rating of the equipment [S2].

For a cost-only illustration, assume a session delivers 20 kWh. Choose an illustrative driver price of 50p/kWh and an electricity purchase price of 25p/kWh. These are assumptions, not current market prices. On a simplified equal-energy basis, the driver charge is 20 × £0.50 = £10. The energy purchase is 20 × £0.25 = £5. The difference is £5 before other costs.

That difference is not net profit. This exercise excludes tax, standing and capacity charges, conversion losses, software, payment fees, maintenance, installation and any revenue share. It does not estimate annual income or payback. Use actual invoices and a consistent tax basis before assessing a business case, and check the treatment with your accountant.

For the fuller method, follow charger ROI and payback and setting a public tariff. Return on investment (ROI) compares a return with the capital invested; it needs more inputs than the energy margin alone.

Why does delivered energy differ from the battery increase?

Decide where each measurement is taken. Electricity is converted during charging: alternating current (AC) equipment relies on conversion inside the vehicle, while direct current (DC) equipment carries out conversion outside it [S3]. Do not substitute a battery display for an invoice meter reading in your costing worksheet.

Ask the proposed installer to identify the site meter, charger meter and vehicle estimate. Request an energy balance that states conversion and auxiliary consumption assumptions. There is no universal loss percentage used in this guide. The important comparison is whether your purchase-cost calculation and sales calculation refer to the same measurement boundary.

What should you check before choosing equipment?

Write your charging requirement as energy by a deadline. Then ask for the average power needed to deliver it. Compare that requirement with the vehicle’s acceptance limit and the building’s available supply [S3], [S4]. For a shared car park, ask for a busy-period example with several vehicles connected.

A distribution network operator (DNO) runs the local electricity network. Discuss connection requirements through your installer. Load management can control charging within a supply limit, but it does so by sharing power rather than creating more of it [S4].

Take your worksheet to the commercial cost guide, the funding comparison and your relevant workplace sector guide. Compare fully funded, part funded and self funded proposals using the same energy requirement.

What should you do next?

Prepare a sample day showing vehicles, arrival times, departure times and energy needed. Then get matched with an installer that covers your area to assess the supply and equipment.

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